The frequency band allocated for a cellular communication system is typically severely limited, and therefore the resource must be effectively divided between remote/mobile stations. A fundamental property of a cellular communication system is that the resource is divided geographically by the division into different cells. An important advantage of a cellular communication system is that due to the radio signal attenuation with distance, the interference caused by communication within one cell is negligible in a cell sufficiently far removed, and therefore the resource can be reused in this cell. In order to optimise the available communication capacity in a cellular communication system, it is advantageous to have the mobile stations distributed over different cells in accordance with the available communication capacity.
A system design based on cells is typically based on an ideal cell pattern. However, an idealised cell pattern never occurs in practice, due to the nature of the physical and propagation environment and the fact that cell sites and antennae are not ideally located on a regular grid pattern. The network designer therefore uses frequency planning tools to estimate the radio propagation for each cell and predict a corresponding coverage area. Based on these propagation models, the network designer is able to develop a frequency plan for the network intended to minimise the expected interference and optimise coexistence between the different cells. The frequency plan considers such factors as antenna heights and location, terrain topology, transmitted power levels, the anticipated number of subscribers, the traffic mix etc.
However, although such frequency plans may provide acceptable performance in many scenarios they also have associated disadvantages. For example, the propagation models used to estimate radio propagation conditions may often be less accurate than desired. Also, even if such propagation estimates are based on previous measurements of radio conditions in a live system, the resulting estimates tend to be typical values reflecting past behaviour. In addition, the centralised nature of frequency planning operations require a large amount of measurement data to be collected centrally which may complicate the operation of the cellular network and use high amounts of the limited bandwidth resource of the network. Furthermore, the derived frequency plan tends to reflect expected or estimated conditions which are typically derived from average conditions determined in the past and are not able to reflect and adapt to the future instantaneous conditions of the system which may deviate significantly from the typical conditions.
A frequently encountered problem is where one or more cells is congested and do not have available resource for supporting additional mobile stations (or is close to being congested). Congestion causes calls to be dropped or the quality to be reduced and it is therefore desirable to reduce the probability of congestion occurring. Often, additional resource may be available in other cells, and therefore many cellular communication systems comprise algorithms attempting to utilise such resource.
Although frequency plans may be used to reduce the probability of such dynamic congestion occurring, it cannot be avoided without dimensioning the system for worst case conditions which would result in excessive cost and complexity of the system. Therefore, cellular communication systems often comprise some form of congestion relief management which operates when congestion occurs in a cell. This management seeks to distribute traffic across different frequency resources and specifically seeks to move traffic to neighbouring cells having available capacity.
For example, current 3rd Generation Partnership Project (3GPP) Standards proposals describe the concept of handing over traffic to suitably qualified neighbors based on congestion overload of the serving cell. Such handover can be based on assessment of the current propagation conditions between the mobile station and the neighbors as well as the current loading of the neighbour base stations. Typically, a call may be handed over to a neighbour cell from a congested cell if the neighbour cell is not congested and the radio propagation conditions are considered sufficiently good to support the call (for example as estimated on the basis of a measured signal level for a pilot signal transmitted by the neighbour cell).
However, although such conventional congestion relief may substantially alleviate the situation it also tends to have some disadvantages. In particular, the conventional approach tends to provide suboptimal performance and to reduce the quality of service experienced by the mobile stations. More specifically, the current congestion relief handover approach is based on a determination of the neighbour cell's current ability to support the call but does not reflect the neighbour cell's ability to continue to support the call. Accordingly, it frequently occurs that a mobile station is successfully handed over to another cell followed by the call being dropped within a relatively short time interval because the new cell cannot continue to support the call (e.g. because the propagation conditions deteriorate or the mobile station moves outside the area supported by the new cell).
Hence, an improved cellular system would be advantageous and in particular a system allowing increased flexibility, reduced complexity, facilitated operation, facilitated implementation, improved congestion relief, improved handover performance, improved continued support of communication services following handovers and/or improved performance would be advantageous.